The aim of this second competitive renewal proposal (years 11-15) is to pursue our program identifying the function of c-Src that, in the osteoclast, is required for bone resorption (src- mutants develop osteopetrosis) and is unique to c-Src (i.e., cannot be compensated by other Src family of kinases in this cell). During the first eleven years of this program, we have made very significant progress towards understanding the functions of c-Src in bone resorption. Although the mechanism(s) by which src deletion leads to a decrease in bone resorption is not yet fully characterized, it is now clear, in part through our work, that Src regulates integrin-dependent osteoclast adhesion and motility. Two central questions remain unanswered: 1/ What is the molecular basis for Src's "uniqueness" in osteoclast function? and 2/ Why would the phosphorylation of Cbl be so important in the osteoclast? For the next funding period, we propose to 1) identify the molecular basis for Src's unique function in osteoclasts, 2) explore the mechanisms by which phosphorylation of Cbl regulates its functions, linking integrins to actin, podosome turnover and osteoclast function and 3) extend these in vitro studies to in vivo models by generating new transgenic mice that express relevant mutated versions of Src or Cbl. The specific aims of this competitive renewal proposal are: 1) To identify the specific domains of Src that are responsible for the osteoclast phenotype (i.e. cannot be compensated by other members of the Src family of non-receptor tyrosine kinases); 2) To further characterize the mechanisms by which Src-mediated phosphorylation of Cbl regulates osteoclast function, i.e. the ability of Cbl to ubiquitinate and/or interact with other molecules [upstream components (Pyk2) and downstream components (dynamin and PI3-kinase)];and 3) To translate the in vitro results obtained during the previous funding period and in Aims 1 and 2 into in vivo studies by creating new transgenic mice that express mutated Src or Cbl proteins that alter osteoclast function in vitro. The work proposed here will increase understanding of the regulation of osteoclast function and bone resorption, potentially identifying novel targets for therapeutic intervention to control bone loss related to osteoporosis and other diseases with increased bone resorption such as bone metastasis and periodontal and joint diseases.